The invention relates to mixed signal processing and more particularly to an analog to digital converter having a negative bias generator providing a negative logic supply rail for powering digital processing circuitry.
Mixed signal processing techniques are known in which an analog signal is converted to digital, processed in digital and then the output is provided in either analog or digital form. Techniques of this nature find frequent application in such diverse fields as audio circuits and industrial instrumentation.
Thermocouples are a type of sensor used in industrial instrumentation. They have two dissimilar metals, welded together, producing almost a dead ohmic short but with a strange, few millivolt temperature-dependent series potential. Thermocouples are sometimes placed in ovens with grounded inside surfaces. If one were to insulate the thermocouple from the inside of the oven, one would slow down its response to temperature transients. If one were required to bias the thermocouple up to a voltage, such as 1.5V, to allow analog signal circuitry such as analog to digital converters operating between 0V and 3V enough headroom to work accurately, and if one does not insulate the thermocouple, there would be a risk of sparks occurring from accidental contact between the thermocouple and the wall of the oven.
One would like to connect one side of the thermocouple to the same potential as the inside surface of the oven. This can be done by operating an ADC between a positive and negative potential (e.g. between xe2x88x921.5V and +1.5V) and allowing the thermocouple to operate at zero V, the same potential as the grounded oven wall. However, one does not wish to buy an extra power supply in order to provide both +1.5V and xe2x88x921.5V to an integrated circuit processing thermocouple signals. A single 3V supply is much cheaper.
Accordingly, it would be desirable if the problems associated with the prior art techniques could be overcome.
The problems of the prior art are overcome, in one embodiment of the invention by providing a negative bias generator to which analog, digital or both analog and digital processing may be referenced.
In another embodiment of the invention, the power consumed by digital processing circuitry in a mixed signal application is minimized by allowing the digital circuitry to operate referenced to a negative logic supply rail (for P substrate technologies) so that devices implementing the digital circuitry can operate at or near an energy or energy-delay minimum.
The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.